Elementary electrochemical cells used to separate oxygen from air, or from a gas mixture containing it, are generally formed from a ternary system consisting of solid electrolyte/electrodes/current collectors.
The solid electrolytes used for separating oxygen from a gas mixture are doped ceramic oxides, which, at the operating temperature, are in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite or brown-millerite cubic phases called aurivillius phases; J. C. Boivin and G. Mairesse have referenced all the O2− anionic conducting crystal phases in a general article (Chem. Mat., 1998, p. 2870-2888, “Recent Material Developments in Fast Oxide Ion Conductors”).
The electrode materials associated with the solid electrolyte are generally perovskites. These are materials possessing a crystal structure of the ABO3 or AA′BB′O6 type (A, A′: lanthanide and/or actinide; B, B′: transition metals) based on the structure of natural perovskite CaTiO3. These materials exhibit good hybrid (ionic/electronic) conductivity properties thanks to this cubic crystal structure, in which the metal ions lie at the corners and at the centre of an elementary cube and the oxygen ions at the middle of the edges of this cube. The electrode materials may also be perovskite material/purely ionic conducting material mixtures or else mixtures based on materials possessing other crystal phases, for example of the aurivillius, brown-millerite or pyrochlore type.
Current is collected either by a metal or a metal lacquer or by a metal/“inert oxide” (such as alumina) ceramic mixture, or by a metal/carbide (such as silicon carbide) mixture or by a metal/nitride (such as silicon nitride) mixture, in which the principle role of the oxide, carbide or nitride is that of mechanically blocking the segregation/sintering phenomena that appear owing to the high operating temperatures (700° C.<T<900° C.), especially when silver is used as current collector metal, or by a metal/“hybrid conductor” oxide ceramic (such as an oxide of the perovskite structure of the family of strontium-doped lanthanum manganites) mixture or by a metal/“ionic conductor” oxide ceramic (such as yttrium-stabilized zirconia) mixture.
However, the Applicant has found that when a tubular electrochemical cell, in which the solid electrolyte is zirconium oxide stabilized with 8 mol % yttrium oxide (8% YSZ), the electrodes are made of La0.9Sr0.1MnO3−δ(LSM) and the current collectors are a silver lacquer, is operated at a temperature of between 700 and 900° C., whether at atmospheric pressure or under an internal oxygen pressure of between 1 and 50×105 Pa (1-50 bar) or under an external oxygen pressure of between 100 and 150×105 Pa (100-150 bar), this cell is observed to undergo accelerated ageing. This is manifested by a 70% increase in the cell voltage in 40 hours of operation.
By replacing the silver lacquer current collectors with 50/50 vol % Ag/(8% YSZ) or 50/50 vol % Ag-LSM “cermet” (metal/ceramic mixtures) current collectors, the rate of ageing is greatly reduced. However, the degradation phenomenon is not completely eliminated as a 6 to 20% increase in the total voltage is observed for 100 hours of operation. When the cell works under an internal oxygen pressure of between 1 and 50×105 Pa (1-50 bar) for temperatures of between 750° C. and 800° C., a lowering of the coulombic efficiency and a drop in the potential may also be observed.
L. S. Wang and S. A. Barnett have described the use of LaCoO3 for coating stabilized zirconia-based cells coated with an Ag/YSZ mixture. These studies have shown that after operating for 150 hours at 750° C., the (50/50) YSZ/Ag-YSZ/LaCoO3 layer system did not lose silver, unlike the system without the “protective” LaCoO3 layer, for which there was segregation and loss of silver mass by evaporation over time. However, the LaCoO3 perovskite does not have good hybrid conductivity properties.
The Applicant started from the assumption that, in the case of silver-lacquer-based current collectors, the ageing or degradation of the system (1<P<50×105 Pa) and the drop in coulombic efficiency under pressure (P>20×105 Pa) and at high temperature (800° C.) were consequences of the poor architecture of the cell used.
The term “architecture” is understood to mean the structures and microstructures of the various constituent materials of the ceramic membrane, namely the solid electrolyte (8 mol % YSZ, yttrium-stabilized zirconia), the electrode (LSM: strontium-doped lanthanum manganite) and the current collector (silver lacquer or silver/oxide or non-oxide ceramic cermet on the cathode side; gold lacquer on the anode side).
The term “structure” is understood to mean the chosen system of stacked layers and the order of the various coatings deposited in order to make up an electrochemical cell (solid electrolyte/electrode/current collector) and the geometrical shapes (tube, plate) of the membranes.
The term “microstructure” is understood to mean the thicknesses, densities, areas and roughness within the various materials characterizing the membrane, the sizes and morphologies of the grains and/or particles of the various materials, the intergranular and intragranular porosity of the solid electrolyte, the nature (morphology) of the surface of the solid electrolyte, the porosity and stacking of particles of the various coatings (electrode, current collector).
The Applicant assumed that, under certain operating conditions (temperature, oxygen pressure, applied current density), the use of silver as current collector results in segregation/sintering of this metal at temperatures above 750° C., in its evaporation, accentuated by hot-air flushing of the cell at temperatures above 700° C. and in its diffusion under pressure (20×105 Pa) through the solid electrolyte at high temperature (>780° C.). Such diffusion could depend not only on the operating conditions allowing the possible presence of silver in quasi-liquid form, but also on the use of ceramic membranes of low density (less than 95%) which have a high intergranular and intragranaular porosity and an unsuitable microstructure, both in terms of grain size of the solid electrolyte and of grain boundaries.
After having observed, after operation, debonding of the electrode/current collector coatings on the internal surface of the membrane, that is to say on the anode side where the oxygen is produced, and debonding of the electrode/current collector interfaces in the cells used hitherto, the Applicant also assumed that this phenomenon could be due to the absence of developed specific surface area and roughness on the internal or external surface of the solid electrolyte.